1. Field of the Invention
The invention relates to a composite adhesive for optical and opto-electronic applications, for example for connecting individual components such as optical fibres or for constructing (integrated) optical chips (IO-chips) for the production of optical fibre-chip couplings etc.
2. Description of the Background
Various UV or thermally curing transparent organic adhesives are presently employed in opto-electronics for connecting components of different materials, e.g., SiO.sub.2 optical fibres and IO-chips made of glass, silicon, lithium niobate or semi-conductors, and for constructing optical components from these materials.
In doing so, utmost precision in adjusting the parts to be connected is required (.+-.0,1 .mu.m) so that e.g., in fibre-chip-couplings the lightwave guides in the fibre and in the chip come into exact contact and light transmission losses are minimized thereby. An additional requirement is that the glued joint must remain stable in the temperature range of from -45.degree. C. to +85.degree. C. (or +135.degree. C., respectively) and does not undergo maladjustment within said range.
Conventional adhesives do not meet said requirements satisfactorily. They often tear at temperatures below -40.degree. C. and due to the relatively high coefficient of thermal expansion of the organic adhesive the glued joint is subject to an excessively high maladjustment or may be destroyed at elevated temperatures.
It is known from the prior art that, e.g., the curing behaviour (shrinkage) of transparent adhesive may be improved by mixing finely ground (particle size in the .mu.m range), already cured adhesive into the uncured adhesive. This, however, inevitably leads to an increase in the viscosity of the adhesive so that the amount of solids that can be added is limited to a few %, depending on the application.
Furthermore, no qualitative change in the properties of the cured composite adhesive (e.g. an increase of the thermal stability or a decrease of the coefficient of thermal expansion) can be achieved in this manner. When using inorganic powder having particle sizes in the .mu.m or sub-.mu.m range the transparency decreases due to light scattering. With certain limitations this also applies to agglomerated powders of nanoscale primary particles (e.g. aerosils).